1. Field of the Invention
The present invention concerns a modulator able to modulate the amplitude of an optical wave or to modify pre-existing amplitude modulation of an optical wave.
2. Description of the Prior Art
In this context modulation means any variation of the amplitude of the wave with time. In the typical situation in which the purpose of the modulation is to enable transmission of information, the information elements can be represented directly by the instantaneous values of this amplitude. However, the modulation can also convert an optical wave into a succession of pulses, in which case the information elements are represented by the gaps between the pulses.
This invention finds applications in certain cases where a modulator has to process optical signals that are particularly brief or fast. This requires the modulator to be equally fast, in other words for it to have equally short response times. Its passband then extends up to the corresponding high frequencies. These situations arise in fiber optic transmission networks in which information to be transmitted is represented by extremely brief timed pulses constituting solitons.
Various types of fast optical modulator are known in the art.
A first type of modulator of this kind is the electro-absorption modulator which includes a reverse-biased semiconductor diode. This diode receives the optical wave to be modulated and an electrical control signal. Its bandwidth is unfortunately limited to around 10 GHz or 20 GHz, which is sometimes insufficient.
This type of modulator is described in an article by F. Devaux et al "Full polarization insensitivity of a 20 Gb/s strained-MQW Electroabsorption Modulator", IEEE Photonics Technology Letters Vol 6 No. Oct. 10, 1994, p. 1203-1206.
A second prior art fast modulator is the semiconductor amplifier including a forward-biased diode. This diode receives the wave to be modulated and an electrical control signal. Its bandwidth is unfortunately limited to around 20 GHz. This type of modulator is described in an article by J. C. Simon et al "Gain and Noise characteristics of a 1.5 .mu.m near travelling wave semiconductor laser amplifier", Electronic Letters vol. 25 No. 7, 1989, p. 434-436.
A third type of prior art fast modulator is the non-linear optical loop mirror (NOLM). It is constructed from optical fibers and uses an optical control signal. This signal is formed optically from pre-existing modulation of the wave to be modulated. More particularly, clock pulses are formed from this wave by an optical clock recovery system and constitute the control signal of the modulator for resynchronizing pulses of the wave to be modulated. This type of modulator is described in an article by S. Bigo, O. Audouin and E. Desurvire "Analysis of soliton in-line regeneration through two-wavelength nonlinear loop mirror as synchronous amplitude/phase modulator", Elect.
Letters, Dec. 7th1995, Vol. 31, No. 25, p. 2191. It has the drawback of being complex, in particular if a pre-existing polarization state of the wave to be modulated must be maintained.
A fourth prior art fast modulator is a surface reflection saturable absorber, i.e. a stack of semiconductor layers including a Bragg mirror and quantum wells. One of the two outside surfaces is metallized to reflect light and the other is transparent. The wave to be modulated and a control wave having a different wavelength and constituting an optical control signal are injected simultaneously through the transparent surface. The wave to be modulated is partially reflected by the Bragg mirror and completely reflected by the metallized surface. Because of the resulting interference in particular, its output power is increased or decreased by increasing or decreasing the power of the control wave. This modulator is described in an article by H. Tsuda, A. Hirano, R. Takahashi, K. Sato and K. Hagimoto: "3 PS, 2.4 Gbit/s All-Optical Pulse Discrimination Experiment: All-optical Regenerator based on a High-speed Saturable Absorber Optical Gate", Proc. 21st Eur. Conf. on Opt. Comm. (ECOC'95-Brussels) pp 949-952. Industrial application of this type of modulation would appear to be inevitably difficult, especially if it has to be included in an optical device integrated on a semiconductor substrate.
The aims of the present invention include:
widening the bandwidth of an optical modulator in a simple manner, PA1 in particular avoiding the use of electrical modulation signals for this purpose, PA1 providing a fast optical modulator preserving the polarization state of a wave to be modulated in a simple manner, PA1 facilitating the incorporation of a fast optical modulator into an integrated optical device, and PA1 providing a simple way to resynchronize solitons in a communication system using solitons guided by long optical fibers.